Embodiments of the present invention pertain generally to implantable medical devices, and more particularly pertain to medical devices that utilize STS segment variations within cardiac signals to characterize a patient's myocardial condition.
Cardiac ischemia is a condition whereby heart tissue does not receive adequate amounts of oxygen and is usually caused by a blockage of an artery leading to heart tissue. Ischemia arises during angina, acute myocardial infarction, coronary angioplasty, and any other condition that compromises blood flow to a region of tissue. When blockage of an artery is sufficiently severe, the cardiac ischemia becomes an acute myocardial infarction (AMI), also referred to as a myocardial infarction (MI) or a heart attack.
Many patients at risk of cardiac ischemia have pacemakers, implantable cardioverter/defibrillators (ISCDs) or other medical devices implanted therein. Cardiac signals can be characterized by electrocardiograms (ECG) and/or intra-cardiac electrograms (IEGM). ECGs and IEGMs are useful for diagnosing ischemia and locating damaged areas within the heart. ECGs are composed are various waves and segments that represent the heart depolarizing and repolarizing. The ST segment (STS) represents the portion of the cardiac signal between ventricular depolarization and ventricular repolarization. While P-waves, R-waves and T-waves may be generally considered features of a surface electrocardiogram (ECG), for convenience and generality, herein the terms R-wave, T-wave and P-wave are also used to refer to the corresponding internal cardiac signal, such as an intra-cardiac electrogram (IEGM).
Techniques have been developed for detecting cardiac ischemia using implanted medical devices. Some conventional IEGM-based ischemia detection techniques seek to detect ischemia by identifying changes in the elevation and/or depression of the STS from the baseline of the cardiac signal that occur during cardiac ischemia. Elevation and/or depression of the STS in a cardiac signal may result when there are abnormalities in the polarizations of cardiac tissue during an acute myocardial infraction (MI). An STS variation arises because of differences in the electric potential between cells that have become ischemic and those cells that are still receiving normal blood flow. Variation of the STS from a baseline is a result of injury to cardiac muscle, changes in the synchronization of ventricular muscle depolarization, drug or electrolyte influences, or the like.
Patients that experience ischemia events may show signs of STS elevation or depression. Conventionally, STS variations found in cardiac signals are captured and displayed in a limited manner. However the conventional presentation of STS variation has provided limited information. For instance, conventional presentation formats show STS variation for near term events (e.g., one to seven days) which are not sufficient indicators of ischemia that develops more slowly over longer periods of time. Thus, conventional presentation formats for STS variation can mask ischemia that develops over a longer period of time. Therefore, a reliable presentation and ischemia detection method and system are needed that are able to measure the STS variation trend over a long period of time and present the trend in a manner that more readily indicates potential changes in ischemia events.